1. Technical Field
The present invention pertains to reception of a frequency-hopped signal. In particular, the present invention pertains to signal detection of a frequency-hopped signal including a frequency shift keying (FSK) sequence in an environment including sinusoidal interference.
2. Discussion of Related Art
A manner of enhancing security and reliability within communication systems includes employment of encoded signals, such as spread spectrum signals. One type of spread spectrum technique is commonly referred to as frequency-hopping. In this type of spread spectrum system, a baseband signal containing the desired information is modulated with a) narrowband carrier signal. The carrier signal is shifted or hopped in frequency based on a spreading or pseudonoise code to spread the modulated signal over a wider bandwidth. The frequency order for transmission of the carrier signal is derived from the code. In order to acquire the spread spectrum signal, a receiving unit is typically required to shift or hop frequencies at a compatible rate in accordance with the spreading code. The frequency-hopped signal may include a frequency shift keying (FSK) sequence (e.g., a sequence employing plural frequencies within the signal) to encode or represent varying states of information. The sequence typically employs alternating frequencies within the signal each displaced or shifted relative to the carrier signal or hop frequency by a frequency offset.
A conventional or legacy receiver for detecting a frequency shift keying sequence (FSK) within a frequency-hopped carrier signal is illustrated in FIG. 1. Specifically, a receiver 10 includes a numerically controlled oscillator (NCO) 12 and a processor 16 to perform signal detection. The frequency of oscillator 12 is adjusted by processor 16 to detect a particular waveform format or sequence within an incoming frequency-hopped signal. The sequence is in the form of a frequency shift keying (FSK) sequence that initially shifts the frequency of the carrier signal or hop by a positive frequency offset and utilizes that frequency (e.g., a hop frequency+1852 Hz) for one-hundred twelve symbol times (Ts), switches to and utilizes a subsequent frequency shifted from the carrier signal or hop frequency by a negative frequency offset (e.g., the hop frequency−1852 Hz) for ninety-six symbol times, reverts back to the initial frequency (e.g., the hop frequency+1852 Hz) for forty-eight symbol times, and subsequently switches to the frequency displaced from the carrier signal or hop frequency by the negative frequency offset (e.g., the hop frequency−1852 Hz) for forty-eight symbol times.
The incoming frequency-hopped carrier signal with the FSK sequence is initially received by the legacy receiver. The received signal and the signal produced by oscillator 12 are provided to processor 16 for processing. In particular, the processor correlates the received and oscillator signals. The correlated output is integrated over sixteen symbol times with the resultant energy being stored. The integration results in nineteen energy values for a hop of three-hundred four symbols (e.g., the sum of the symbols within each frequency shift of the carrier signal FSK sequence (112+96+48+48=304 symbols), where each energy level is integrated over sixteen symbol times, thereby producing nineteen energy levels (304/16=19) for the sequence). When the correlation is viewed as a filter, integration over sixteen symbols, rather than the entire three-hundred four symbols, within the hop widens the bandwidth. This enables some frequency offset in the received signal to be tolerated without causing missed signal detections. The frequency response of the sixteen symbol “boxcar” type filter corresponding to the symbol integration process is illustrated in FIG. 2.
Once the energy values are determined, the energy measurements or values corresponding to locations adjacent (e.g., on either side of) the frequency transition points or shifts of the FSK sequence (e.g., the end of a previous frequency shift and the beginning of the next frequency shift) are discarded as illustrated in FIG. 3 (e.g., the discarded energy values are indicated by the character ‘X’ as viewed in the figure, where the energy values represent integration of sixteen symbols as described above). These energy values are discarded since the values may be contaminated by data with an incorrect frequency value due to the exact time of arrival (TOA) of the received signal not being precisely known.
The energy values associated with the same frequency are combined to form respective summations (e.g., a summation corresponding to energy measurements associated with the hop frequency+1852 Hz, a summation corresponding to energy measurements associated with the hop frequency−1852 Hz, etc.). The greater summation is determined and utilized to produce a normalization of the lesser summation. The normalization is basically the result of the lesser summation divided by the greater summation. When the normalized summation exceeds a threshold value, the signal is considered detected. The normalization of the lesser summation is required to prevent interference near certain frequencies (e.g., the hop frequency+1852 Hz and the hop frequency−1852 Hz) from causing a false detection in the absence of a valid signal.
The legacy receiver suffers from several disadvantages. In particular, the legacy receiver signal detection fails to detect valid signals in the presence of sinusoidal interference. The measured level and frequency where sinusoidal interference causes the legacy receiver signal detection to fail for a signal level of approximately −6 dBm is illustrated in FIG. 4. As viewed in this figure, the level and frequency where the legacy receiver signal detection fails mirrors the frequency response of the sixteen symbol “boxcar” type filter (FIG. 2). The legacy receiver detection is vulnerable near the hop frequency+1852 Hz and at the sidelobes of the boxcar type filter (e.g., near the hop frequency±500 Hz). The reason for the vulnerability is due to the interfering signal affecting one side of the frequency shift keying sequence (e.g., the hop frequency+1852 Hz or the hop frequency−1852 Hz). Accordingly, the normalization of the lesser energy summation suppresses that summation (e.g., the greater summation is enhanced by the interference), thereby preventing the normalized summation from exceeding the threshold level. Further, the legacy receiver signal detection discards energy values, thereby increasing vulnerability to noise and providing greater quantities of missed signal detections and false alarms.